Optical fiber thermal property probe

ABSTRACT

An optical fiber sensor extends coaxially with a controllable heater to provide high-resolution axial measurement of thermal properties such as thermal convection of the surrounding, Heat removal by either conduction or convection may be used to deduce material height in a tank, or velocity of flow when coupled with localized heating, or other aspects of the material based on thermal conductivity.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No. 62/381,324 filed Aug. 30, 2016 and hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical fiber probe, and inparticular, to an optical fiber probe that can sense thermal propertiesof the surrounding medium.

Next generation (generation IV) nuclear reactors such as the Sodium FastReactor (SFR) employ alkali liquid metal (e.g., sodium) as a coolantcarrying heat from the reactor core. Critical for the construction ofsuch reactors is an ability to provide instrumentation that can measurelevel, temperature, and velocity distributions in a material such asliquid sodium or fuel at temperatures above 371 Kelvin.

Normal instrumentation, for example, for level sensing, includingultrasonic and mechanical approaches, can be prone to failure in suchextreme operating environments,

SUMMARY OF THE INVENTION

The present invention provides an optical fiber linked with a heaterallowing it to characterize thermal properties of nearby materials atmultiple points along the fiber at extremely high temperaturesassociated with nuclear reactors or heat storage systems. Measurementsby the system may determine material levels (such as the level of liquidsodium or similar transfer or storage material) by detecting changesbetween the thermal properties of the material and an overlying gascolumn, or measure rates of flow when coupled with localized heating, orcharacterize nuclear fuel during use or the like.

Specifically, the invention provides a sensor system for characterizingmaterial thermal properties and includes an optical fiber, a heaterstrip coextending with the optical fiber, an optical assembly fortransmitting light into the optical fiber and detecting light receivedfrom the optical fiber, and an electronic circuit communicating with theoptical assembly and the heater strip to: (a) characterize temperatureat multiple points along a length of the optical fiber from the lightreceived from the optical fiber as influenced by the heat output fromthe heater strip; and (b) determine and output a measure related tothermal property of a medium surrounding the optical fiber and heaterstrip.

It is thus a feature of at least one embodiment of the invention toprovide sophisticated multipoint analysis of thermal conductivity ofmaterials such as can be used for a variety of sensing purposes.

The sensor system may include a thermally conductive shield around theoptical fiber.

It is thus a feature of at least one embodiment of the invention topermit use of the sensor system in hostile environments such as inliquid sodium, molten salts, or other organic and inorganic substances.

The conductive shield may be a stainless-steel tube filled with a fluidconductive medium positioned between an inner wall of thestainless-steel tube and the optical fiber.

It is thus a feature of at least one embodiment of the invention toprovide robust shielding of the optical fiber that prevents thermalstresses caused by engagement between the fiber and the tube whilepromoting high thermal conductivity.

The conductive medium may be a moisture-free gas such as helium and theconductive shield may be hermetically sealed.

It is thus a feature of at least one embodiment of the invention toprovide a shielding system that can accommodate extreme temperaturevariations without damage to the optical fiber.

The sensor system may provide a sheath surrounding the optical fiber andthe heater strip including a conductive potting material holding theheater strip and optical fiber in thermal proximity.

It is thus a feature of at least one embodiment of the invention toprovide a robust assembly that can produce consistent measurementresults by close thermal linking between the heater and the opticalfiber.

The heater strip may be a resistive wire element providing substantiallyuniform distributed heating along the length of the heater strip.

It is thus a feature of at least one embodiment of the invention toprovide a uniform and constant heating to simplify the determination ofthermal properties such as thermal conductivity or thermal convection.

These particular objects and advantages may apply to only someembodiments falling within the claims and thus do not define the scopeof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a level sensor for liquid heatstorage material employing the present invention providing a verticallyextending probe passing through a surface of the liquid heat storagematerial and communicating with a reflectometer system;

FIG. 2 is a fragmentary perspective view of the probe of FIG. 1 showingparallel extending optical fiber and heater elements within a probesheath;

FIG. 3 is a fragmentary cross-section of the probe of FIG. 2 showing theuse of the heater element to reveal thermal conductivity of theenvironment of the probe;

FIG. 4 is a signal flow diagram describing a program executed by thereflectometer of FIG. 1;

FIG. 5 is a fragmentary figure similar to FIG. 1 showing the use of theprobe for flow velocity sensing;

FIG. 6 is a fragmentary view similar to that of FIG. 4 showingcomparison of temperatures at different times to deduce flow velocity;

FIG. 7 is a fragmentary view of the signal flow diagram of FIG. 4showing a curve-fitting process;

FIG. 8 is a figure showing curvature of the probe to provide multiplemeasurement points over an area and/or volume;

FIG. 9 is a fragmentary view of an alternative embodiment of the probein which the fiber optic shield provides the heater element; and

FIG. 10 is an alternative application of the probe for providingmultipoint flow measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a sensor system 10 of the present invention maybe used, for example, to determine a height of a surface level 12 of aheat storage material 14 such as liquid sodium or other liquid metals orsalts such as but not limited to NaNO₃KNO₃ salts and FLIBE salts(lithium fluoride and beryllium fluoride) up to 600 C. This heat storagematerial 14 may be held within an accumulator 16, for example, theaccumulator 16 providing a vertically extending enclosed containercommunicating through inlet 18 and outlet 20 at the upper and lower endsof the accumulator 16 with elements of a nuclear reactor or the like.The sensor system 10 includes a vertically extending probe 22, forexample, attached to penetrate an upper wall of the accumulator 16through a sealing fitting 24 and pass downward through the surface level12 into the heat storage material 14 within the accumulator 16.

A proximal end of the probe 22 outside of the accumulator 16 may connectto a reflectometer system 26 through optical conductor 28 (such as anoptical fiber). Optical conductor 28 may be received by an opticalassembly 32 (as will be described below) communicating in turn with anelectronic computer 34. The proximal end of the probe 22 may alsoreceive electrical signals communicated through electrical conductor 30with a controllable power source 36 controlled by the computer 34.

Computer 34 may include one or more processors 38 executing a program40, as will be discussed below, held in standard computer memory 42. Thecomputer 34 may provide an output 44, for example, communicating with adisplay 46 which may display a number indicating a height of the liquidheat storage material 14 in the accumulator 16 or, as will be discussedin a further embodiment below, a velocity of movement of the fluid 14 orother thermal characteristics of material in contact with the probe 22Alternatively, it will be appreciated that the display may indicateother thermal properties such as convection, thermal conductivity or thelike and may be in a variety of different units or unitless.

A reflectometer system 26 providing the optical assembly 32 andoptionally portions of the electronic computer 34 and suitable for usewith the present invention is commercially available from Luna ofRoanoke, Va., USA, under the tradename of Luna ODiSI-B OpticalDistributed Sensor Interrogator.

Referring now also to FIG. 2, the probe 22 may provide for a generallylinearly extending outer sheath 50, for example, in the form of astainless-steel tube. Positioned within the outer sheath 50 andextending therealong may be an elongate heater element 54 and an opticalfiber assembly 56, each generally adjacent to and parallel to the otherand axially aligned within the sheath 50 to extend along a length of thesheath by a distance sufficient to pass above and below the surfacelevel 12 over an expected range of heights of surface level 12. Theelongate heater element 54 and optical fiber assembly 56 may be fixedwithin the outer sheath 50 in a thermally conductive potting material52.

The heater element 54, for example, may provide for an outer tubularsheath 58 of thermally conductive material (for example, metal) holdinga resistive heating element 60 (for example, a high resistanceelectrical conductor such as nichrome) extending downward along thelength of the probe 22 and looping backward to provide a completeelectrical circuit. The resistive heating element 60 provides ohmicheating uniformly along its length and along the length of the opticalfiber assembly 56 although predictable variability in ohmic heatingalong the length of the resistive heating element 60 can beaccommodated. The resistive heating element 60 may connect viaelectrical conductor 30 to receive a constant current electrical flowfrom the controllable power source 36. A space between the resistiveheating element 60 and the tubular sheath 58 may be filled with anelectrically insulating but thermally conductive potting material 62 forexample a hardening ceramic powder or other material that can toleratethe necessary heat range. In alternative embodiments, the heatingelement 60 may extend downward through the probe 22 and the returnelectrical path may be provided through the sheath 50 or by separateconductor.

The optical fiber assembly 56 may provide for thermally conductive outersheath 64, for example, a stainless-steel or ceramic tube hermeticallysealed at its lower and upper ends to retain within the sheath 64atmosphere of moisture-free helium 65 or other highly thermallyconductive gas. Ideally, the material within the sheath 64 will have athermal conductivity equal or exceeding that of helium.

Positioned within the helium 65 is single mode optical fiber 66 having acenter core and outer cladding of different refractive indexes topromote internal reflection. Significantly, the optical fiber 66 may beentirely glass material without a polymer or other coating normally usedin such fibers such as could interfere with measurements at hightemperatures, for example, by presenting a substantially differentcoefficient of expansion. In an alternative embodiment, a graphitecoated fiber may be employed and polymer materials may be used for lowtemperature applications.

A lower end of the optical fiber 66 may be spliced to a dispersive fiberelement 68, for example, having only core material and no outer claddingto promote light leakage therefrom and thereby to eliminate highamplitude reflections from the end of the optical fiber 66.Alternatively, the dispersive fiber elements 68 may be implemented usinga portion of the optical fiber 66 curled into a tight radius to promotesuch light leakage.

As noted, the thermally conductive outer sheath 64 is hermeticallysealed to prevent loss or contamination of the moisture-free helium 65and to shield the optical fiber 66 from moisture. The optical fiber 66is generally mounted, for example, only at the top end of the thermallyconductive outer sheath 64 so as to prevent the introduction of stressin the fiber as the optical fiber 66 and thermally conductive outersheath 64 expand and contract at different rates over the range ofoperating temperatures. In this way, the surrounding moisture-freehelium 65 allows the necessary slippage between the optical fiber 66 andthe sheath 64 in contrast, for example, to a rigid potting material

It will be appreciated that other materials than moisture-free helium 65or other highly thermally conductive gases may be used such as providefreedom of movement of the optical fiber 66 and high thermalconductivity, including metals that retain a liquid state at roomtemperature.

Referring now to FIG. 9, in an alternative embodiment, the heaterelement 54 may make use of the thermally conductive outer sheath 64 toprovide an extremely low diameter probe 22 and one which reduces thethermal mass of the probe and improves coupling of the heater to thesurrounding medium providing faster response. As depicted, the thermallyconductive outer sheath 64 may be, for example, stainless steel havingan electrical contact point 67 a at its proximal end and an electricalcontact point 67 b at its distal end communicating with the electricalconductors 30 allowing a current to be passed along the length of thethermally conductive outer sheath 64 to produce ohmic heating. A similarapproach may be used with a ceramic tube used for the thermallyconductive outer sheath 64, for example, by applying a conductivematerial such as a PTC coating to the thermally conductive outer sheath64. In that case, two parallel separate conductors may be used to allowa return path along the thermally conductive outer sheath 64.

Referring now to FIG. 3, the optical fiber assembly 56 may provide ameasure of temperatures at multiple points along the optical fiberassembly 56 using reflectometer techniques as will be discussed below.When these temperature measurements are combined with an activation anddeactivation of the heater element 54, thermal properties of theenvironment of the probe 22 may be determined. For example, by comparinga change in temperature at a given location along the optical fiberassembly 56 between times of activation and deactivation of the heaterelement 54, thermal conductivity of the surrounding material can bededuced. “Thermal properties” include actual thermal resistance andapparent thermal resistance (for example, influenced by convective flow,velocity, or the like). Thermal properties may be used to deducematerial characteristics, interfaces between different states ofmaterial or different material types, flow velocity, convection, and thelike.

Consider, for example, the environment of liquid heat storage material14 below the surface level 12 and heated air 70 above the surface level12, When the heater element 54 is not activated, the temperaturesmeasured by the optical fiber assembly 56 above and below the surfacelevel 12 may be the same, reflecting a steady-state equilibrium intemperatures between the heat storage material 14 and the air 70.Activation of the heater element 54, however, may raise the temperatureof the optical fiber assembly 56 in the region of the air 70 by morethan the temperature is raised at the optical fiber assembly 56 in theregion of liquid heat storage material 14 resulting from a greaterthermal conductivity or thermal convection provided by the liquid heatstorage material 14. A temperature difference (delta T) 82 measured atmultiple points along the length of the optical fiber assembly 56 withand without activation of the heater element 54 may thus revealfundamental properties of the surrounding materials. More sophisticatedmeasurements may look at the rise time or decay time of temperature whenthe heater is activated and deactivated and for example fit thesemeasurements to a curve indicating thermal conductivity.

Referring now also to FIG. 4, the optical assembly 32 may provide for afrequency controllable laser 72 operated by the computer 34, forexample, to sweep through light frequencies. This swept light isprovided to beam splitter 74 coupling the light into the optical fiber66.

Within the optical fiber 66, Rayleigh scattering in microscopicinclusions within the optical fiber 66 may cause reflections backthrough the optical fiber 66 through the beam splitter 74 to be receivedby photodetector 76. Using principles of optical reflectometery, thetime domain reflection signals 78 can be converted to distancemeasurements (by an inverse Fourier transform) indicating a distancealong the optical fiber 66 of the various inclusions. Knowing thesedistances allows determination of slight changes in the locations of themicroscopic inclusions within the optical fiber (caused by expansion andcontraction of the optical fiber 66 with temperature, for example, bycross correlation between frequency domain reflection signals.

The changes in the locations of the inclusions can be related totemperatures along the optical fiber 66 through knowledge of theexpansion coefficient of the glass of the optical fiber 66 to producetemperature profiles 80 a and 80 b describing the temperature atmultiple points along the optical fiber 66, with temperature profiles 80a and 80 b indicating, for example, temperatures obtained while theheater element 54 is deactivated and activated, respectively.

At each location, the difference between signals 80 a and 80 b can beused to produce a measure of thermal resistivity 82 in the materialadjacent to the probe 22 at that location with lower temperaturedifferences indicating a higher thermal conductivity material. Byapplying a threshold 84 to this measure of thermal resistivity 82, aliquid height 86 may be displayed, for example, on display 46 indicatingthe location of the surface level 12.

Alternatively, it will be appreciated that the heater element 54 mayremain activated and a single signal 80 may be acquired providingtemperatures that reflect an underlying convection or thermalconductivity at the various points along the optical fiber 66.

Referring now momentarily to FIG. 7, instead of using a fixed thresholdas shown in FIG. 4, the measured thermal resistivity 82. (reflectingapparent thermal resistance, possibly caused by convection) may be fitto an empirically or theoretically established curve 85 indicatingexpected apparent thermal resistivity values on either side of a surfacelevel 87 for known materials (for example air and sodium 14). By using acurve fitting process, the determined level 87 indicates the surfacelevel 12 and may be based broadly on multiple points of the thermalresistivity 82 thus providing a more robust and noise resistant levelmeasurement.

In order to provide high axial resolution, radial thermal conductivitythrough the probe 22 or convection from the probe should be promotedrelative to axial conductivity such as can be provided by using a smalldiameter probe, for example, less than 10 millimeters in diameter andtaking additional steps to reduce the thermal resistance radiallycompared to the thermal resistance axially for example by using a thinsheath 58 and minimizing separation between the resistive heatingelement 60 the outer sheath 50 and the optical fiber 66 and/or usingnon-isotropic conductive potting media 52, for example having insulatingregion space axially along the probe 22.

Referring now to FIGS. 4 and 5, in an alternative embodiment, the probe22 may be employed to measure a flow rate of liquid heat storagematerial 14 through the accumulator 16, for example, from the inlet 18to the outlet 20. For this purpose, electrically isolated diametricelectrodes 100 a and 100 b are inserted through the accumulator 16 at aknown location along the axis of flow to contact the electricallyconductive liquid heat storage material 14. The electrodes 100 may beconnected to a power supply 102 controllable by the computer 34 topromote an intense current flow between the electrodes 100 through theliquid heat storage material 14 for a short period of time to create aheated zone 104. Flow of the liquid heat storage material 14 causes theheated zone 104 to progress along the length of the probe 22.

Alternatively, natural fluctuations in the variation in the temperatureof the liquid heat storage material 14 may be measured without heatingby the electrodes 100. These variations may be detected at multiplepoints along the probe 22 to generate a temperature profile that can betracked by correlation with later measurements to deduce movement of theliquid heat storage material 14.

Referring also to FIG. 6, in this case, successive pairs of temperatureprofiles 80 a and 80 b may be obtained not associated with activationand deactivation of the heater but simply at different times. Thesetemperature profiles 80 can be analyzed to detect the passage of theheated zone 104 manifested as a pulsed temperature increase 106 in eachof the profiles 80. By timing the passage of the temperature increase106 between two different locations indicated by the profiles 80, avelocity signal 108 may be output, for example, on display 46.

Referring now to FIG. 8, the use of an optical fiber 66 allows the probe22 to navigate curves up to the limit of flexibility of the materialsand ability of the optical fiber 66 to retain internal reflection.Typically, curvature radii 90 of as little as one inch in radius or lessthan five inches in radius may be tolerated with the presently describedmaterials. As a result, the probe 22 may pass in a serpentine path, forexample, through a tank 92 holding materials to be characterized.

Referring to FIG. 10, the probe 22 may be placed in a conduit 94 toextend generally perpendicular to a flow direction 96 of material withinthe conduit. The contained heater element of the probe 22 thus allowsflow to be measured at multiple points along the length of the probe 22.

It will be appreciated that this invention is not limited to the usewith liquid sodium and in some embodiments may be used with conventionalmaterials in non-extreme temperatures, for example, for flow measurementor height measurement. In one embodiment, the system may be used tomonitor degradation of solid fuel elements within a nuclear reactor byembedding the probe 22 in that material. It will further be appreciatedthat the probe 22 need not be straight but can accommodate gentle bendslimited only by the light retention ability of the optical fiber 66 andthe flexibility of the surrounding protective elements.

Certain terminology is used herein for purposes of reference only, andthus is not intended to be limiting. For example, terms such as “upper”,“lower”, “above”, and “below” refer to directions in the drawings towhich reference is made, Terms such as “front”, “back”, “rear”, “bottom”and “side”, describe the orientation of portions of the component withina consistent but arbitrary frame of reference which is made clear byreference to the text and the associated drawings describing thecomponent under discussion, Such terminology may include the wordsspecifically mentioned above, derivatives thereof, and words of similarimport. Similarly, the terms “first”, “second” and other such numericalterms referring to structures do not imply a sequence or order unlessclearly indicated by the context.

When introducing elements or features of the present disclosure and theexemplary embodiments, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of such elements orfeatures. The terms “comprising”, “including” and “having” are intendedto be inclusive and mean that there may be additional elements orfeatures other than those specifically noted. It is further to beunderstood that the method steps, processes, and operations describedherein are not to be construed as necessarily requiring theirperformance in the particular order discussed or illustrated, unlessspecifically identified as an order of performance. It is also to beunderstood that additional or alternative steps may be employed.

References to “a microprocessor” and “a processor” or “themicroprocessor” and “the processor,” can be understood to include one ormore microprocessors that can communicate in a stand-alone and/or adistributed environment(s), and can thus be configured to communicatevia wired or wireless communications with other processors, where suchone or more processor can be configured to operate on one or moreprocessor-controlled devices that can be similar or different devices.Furthermore, references to memory, unless otherwise specified, caninclude one or more processor-readable and accessible memory elementsand/or components that can be internal to the processor-controlleddevice, external to the processor-controlled device, and can be accessedvia a wired or wireless network.

It is specifically intended that the present invention not be limited tothe embodiments and illustrations contained herein and the claims shouldbe understood to include modified forms of those embodiments includingportions of the embodiments and combinations of elements of differentembodiments as come within the scope of the following claims. All of thepublications described herein, including patents and non-patentpublications, are hereby incorporated herein by reference in theirentireties.

What we claim is:
 1. A sensor system comprising: a glass optical fiber;a heater strip coextending with the glass optical fiber; an opticalassembly for transmitting light into the glass optical fiber anddetecting light received from the glass optical fiber; and an electroniccircuit communicating with the optical assembly and the heater strip to:(a) characterize temperature at multiple points along a length of theglass optical fiber from the light received from the glass optical fiberas influenced by heat output from the heater strip; and (b) determineand output a measure related to a thermal property of a mediumsurrounding the glass optical fiber and heater strip; wherein theelectronic circuit characterizes locations of Rayleigh scatterers withinthe glass optical fiber and correlates them to known temperaturelocations to deduce temperature and determines temperature differencewith activation of the heater strip to determine thermal properties;further including a gas tight thermally conductive shield around theglass optical fiber, the thermally conductive shield providing a fluidcontacting and surrounding the glass optical fiber to allow relativeslippage between the thermally conductive shield and the glass opticalfiber, the thermally conductive shield, fluid and the glass opticalfiber cooperating to prevent stress in the glass optical fiber caused byrelative differences in thermal expansion from affecting thecharacterization of temperature, the fluid being non-reactive with theglass optical fiber.
 2. The sensor system of claim 1 wherein thethermally conductive shield is a tube of a material selected from thegroup consisting of stainless steel and ceramic, and wherein the fluidis positioned between and inner wall of the tube and the glass opticalfiber.
 3. The sensor system of claim 2 wherein the fluid is amoisture-free noble gas.
 4. The sensor system of claim 3 wherein themoisture-free noble gas is helium.
 5. The sensor system of claim 2wherein the fluid is a liquid metal.
 6. The sensor system of claim 2wherein the fluid is a fluid at room temperature.
 7. The sensor systemof claim 1 further including a sheath surrounding the glass opticalfiber and the heater strip including a conductive potting materialholding the heater strip and glass optical fiber in thermal proximity.8. The sensor system of claim 1 wherein the heater strip is a resistivewire element providing substantially uniform distributed heating alongthe length of the heater strip.
 9. The sensor system of claim 1 whereinthe optical assembly is attached to a proximate end of the glass opticalfiber, and a distal end of the glass optical fiber attaches to a lossyoptic segment suppressing reflectance.
 10. The sensor system of claim 1wherein the glass optical fiber is a glass, single-mode fiber, free oforganic cladding material.
 11. The sensor system of claim 1 wherein theoptical assembly provides a variable wavelength laser and a photodetector positioned at a distal end of the glass optical fiber totransmit light into the glass optical fiber through a beam splitter andreceive light out of the glass optical fiber through the beam splitter.12. The sensor system of claim 11 wherein the electronic circuitprovides a Fourier transform reflectometer characterizing temperature bymonitoring Rayleigh scattering from inclusions in the glass opticalfiber as modified by a change in temperature of the glass optical fiber.13. The sensor system of claim 11 wherein the electronic circuitdetermines a material interface by fitting a curve to the determinedthermal properties, the curve providing example thermal propertiesexpected for particular materials at a material interface.
 14. Thesensor system of claim 1 further including a tank holding a liquid heatstorage material selected from the group consisting of a liquid metaland a liquid salt and wherein the glass optical fiber of the sensorsystem extends through a surface boundary of the liquid heat storagematerial and wherein the output of the electronic circuit provides aliquid level height of the liquid heat storage material.
 15. The sensorsystem of claim 1 wherein the probe is flexible to allow operation witha curvature of the probe having a radius of curvature less than 5inches.
 16. The sensor system of claim 1 wherein the fluid has a thermalconductivity no less than that of helium.
 17. The sensor system of claim1 wherein the thermally conductive shield provides electrical contactsallowing passage of current through an electrically conductive materialof the thermally conductive shield providing the heater strip.